Foamed thermoplastic materials are used in many applications including, but not limited to, cushioning and packaging materials, insulation, foamed extruded profiles and injection molded structural foam. Foamed thermoplastic materials offer numerous advantages over other structural materials including thermal and sound insulation, reduced weight per unit volume and low cost per unit volume.
Various methods are known for producing cellular thermoplastic articles which are generally referred to as "foam" or "foamed thermoplastic" articles. Foaming is typically accomplished by injecting an inert gas such as nitrogen or carbon dioxide under high pressure into a molten polymer followed by a release of pressure to expand and create the foamed article. Addition of volatile liquids such as chloro-fluorocarbons and pentane are commonly employed to foam specific polymers including polystyrene and various polyurethanes.
Another group of blowing agents, typically referred to as chemical blowing agents, decompose upon heating to generate gas which is utilized to expand the polymer. Chemical blowing agents are commonly employed in the extrusion of profiles and foam injection molding of automotive parts. Chemical blowing agents utilized for foaming polymers include both inorganic materials (e.g., sodium bicarbonate and sodium borohydride) and organic materials (e.g., azodicarbonamide, tetrazoles and semi-carbazides). Azodicarbonamide, which decomposes in the range of 400.degree. to 450.degree. C., is a commonly used chemical blowing agent which generates nitrogen and carbon monoxide upon decomposition. P-toluenesulfonyl semi-carbazide and 5-phenyltetrazole, having decomposition temperatures ranging from 440.degree.-450.degree. F. and 460.degree.-500.degree. F., respectively, are utilized in the foaming of various high temperature engineering polymers. Variants of 5-phenyltetrazole are stated to be useful as foaming agents in processes wherein processing temperatures approach 750.degree. F. Further information on foaming agents is provided in Modern Plastics Encyclopedia (R.L. Heck. III p. 146. 1985).
In the foaming of molten thermoplastic resins, volatilizable blowing agents form bubbles around nuclei within the molten polymer resin. While a certain number of nuclei are inherently present within the molten polymer, additional nucleating agents may by added to facilitate the production of fine-celled foams. Examples of materials employed as nucleating agents include surfactants, gas-liberating thermally decomposable solids, non-decomposable inorganic solids and materials which provide hot spots such as particulate inorganic solids (e.g., talc).
In the production of cellular thermoplastic articles wherein a molten thermoplastic resin is expanded utilizing a dissolved volatilizable blowing agent, bubble formation takes place in the melt as the pressure on the melt is reduced. Typically, such a pressure drop takes place as the dissolved foaming agent within the melt, residing at a higher temperature and pressure, passes through a die or orifice to the outside of the die or orifice which typically resides at ambient pressure and temperature. The dissolved foaming agent diffuses into the bubble as a bubble is initiated at a nucleating site. Bubble formation is facilitated by the presence of an effective amount of nucleation sites. If such sites are present bubbles will be formed when the driving force is relatively low whereas high driving forces are required when nucleation sites are relatively few in number.